Partial pressure

In a mixture of gases, each gas has a partial pressure which is the hypothetical pressure of that gas if it alone occupied the volume of the mixture at the same temperature.[1] The total pressure of an ideal gas mixture is the sum of the partial pressures of each individual gas in the mixture.

The partial pressure of a gas is a measure of thermodynamic activity of the gas's molecules. Gases dissolve, diffuse, and react according to their partial pressures, and not according to their concentrations in gas mixtures or liquids.

This general property of gases is also true of chemical reactions of gases in biology. For example, the necessary amount of oxygen for human respiration, and the amount that is toxic, is set by the partial pressure of oxygen alone. This is true across a very wide range of different concentrations of oxygen present in various inhaled breathing gases or dissolved in blood.

The total pressure of a mixture of gases is equal to the sum of the partial pressures of the individual gases in the mixture as stated by Dalton's law.[2] This is because ideal gas molecules are so far apart that they don't interact with each other. Most actual real-world gases come very close to this ideal. For example, given an ideal gas mixture of nitrogen (N2), hydrogen (H2) and ammonia (NH3):

Ideally the ratio of partial pressures equals the ratio of the number of molecules. That is, the mole fraction of an individual gas component in an ideal gas mixture can be expressed in terms of the component's partial pressure or the moles of the component:

and the partial pressure of an individual gas component in an ideal gas can be obtained using this expression:

where:

= mole fraction of any individual gas component in a gas mixture

= partial pressure of any individual gas component in a gas mixture

= moles of any individual gas component in a gas mixture

= total moles of the gas mixture

= total pressure of the gas mixture

The mole fraction of a gas component in a gas mixture is equal to the volumetric fraction of that component in a gas mixture.[3]

Vapour pressure is the pressure of a vapour in equilibrium with its non-vapour phases (i.e., liquid or solid). Most often the term is used to describe a liquid's tendency to evaporate. It is a measure of the tendency of molecules and atoms to escape from a liquid or a solid. A liquid's atmospheric pressure boiling point corresponds to the temperature at which its vapour pressure is equal to the surrounding atmospheric pressure and it is often called the normal boiling point.

The higher the vapour pressure of a liquid at a given temperature, the lower the normal boiling point of the liquid.

The vapour pressure chart displayed has graphs of the vapour pressures versus temperatures for a variety of liquids.[5] As can be seen in the chart, the liquids with the highest vapour pressures have the lowest normal boiling points.

For example, at any given temperature, methyl chloride has the highest vapour pressure of any of the liquids in the chart. It also has the lowest normal boiling point (-24.2 °C), which is where the vapour pressure curve of methyl chloride (the blue line) intersects the horizontal pressure line of one atmosphere (atm) of absolute vapour pressure.

It is possible to work out the equilibrium constant for a chemical reaction involving a mixture of gases given the partial pressure of each gas and the overall reaction formula. For a reversible reaction involving gas reactants and gas products, such as:

the equilibrium constant of the reaction would be:

where:

= the equilibrium constant of the reaction

= coefficient of reactant

= coefficient of reactant

= coefficient of product

= coefficient of product

= the partial pressure of raised to the power of

= the partial pressure of raised to the power of

= the partial pressure of raised to the power of

= the partial pressure of raised to the power of

For reversible reactions, changes in the total pressure, temperature or reactant concentrations will shift the equilibrium so as to favor either the right or left side of the reaction in accordance with Le Chatelier's Principle. However, the reaction kinetics may either oppose or enhance the equilibrium shift. In some cases, the reaction kinetics may be the overriding factor to consider.

Gases will dissolve in liquids to an extent that is determined by the equilibrium between the undissolved gas and the gas that has dissolved in the liquid (called the solvent).[6] The equilibrium constant for that equilibrium is:

= partial pressure of gas in equilibrium with a solution containing some of the gas

= the concentration of gas in the liquid solution

The form of the equilibrium constant shows that the concentration of a solute gas in a solution is directly proportional to the partial pressure of that gas above the solution. This statement is known as Henry's Law and the equilibrium constant is quite often referred to as the Henry's Law constant.[6][7][8]

where is also referred to as the Henry's Law constant.[9] As can be seen by comparing equations (1) and (2) above, is the reciprocal of . Since both may be referred to as the Henry's Law constant, readers of the technical literature must be quite careful to note which version of the Henry's Law equation is being used.

Henry's Law is an approximation that only applies for dilute, ideal solutions and for solutions where the liquid solvent does not react chemically with the gas being dissolved.

For example, at 50 metres (164 ft), the total absolute pressure is 6 bar (600 kPa) (i.e., 1 bar of atmospheric pressure + 5 bar of water pressure) and the partial pressures of the main components of air, oxygen 21% by volume and nitrogen 79% by volume are:

ppN2 = 6 bar × 0.79 = 4.7 bar absolute

ppO2 = 6 bar × 0.21 = 1.3 bar absolute

where:

ppi

= partial pressure of gas component i = in the terms used in this article

P

= total pressure = in the terms used in this article

Fi

= volume fraction of gas component i = mole fraction, , in the terms used in this article

ppN2

= partial pressure of nitrogen = in the terms used in this article

ppO2

= partial pressure of oxygen = in the terms used in this article

The minimum safe lower limit for the partial pressures of oxygen in a gas mixture is 0.16 bars (16 kPa) absolute. Hypoxia and sudden unconsciousness becomes a problem with an oxygen partial pressure of less than 0.16 bar absolute. Oxygen toxicity, involving convulsions, becomes a problem when oxygen partial pressure is too high. The NOAA Diving Manual recommends a maximum single exposure of 45 minutes at 1.6 bar absolute, of 120 minutes at 1.5 bar absolute, of 150 minutes at 1.4 bar absolute, of 180 minutes at 1.3 bar absolute and of 210 minutes at 1.2 bar absolute. Oxygen toxicity becomes a risk when these oxygen partial pressures and exposures are exceeded. The partial pressure of oxygen determines the maximum operating depth of a gas mixture.